*09:17* [Pub][ePrint]
A Related-Key Chosen-IV Distinguishing Attack on Full Sprout Stream Cipher, by Yonglin Hao
Sprout is a new lightweight stream cipher proposed at FSE 2015.According to its designers, Sprout can resist time-memory-data trade-off (TMDTO) attacks with small internal state size.

However, we find a weakness in the updating functions of Sprout and propose a related-key chosen-IV distinguishing attack on full Sprout.

Our attack enable the adversary to distinguish detect non-randomness on full 320-round Sprout with a practical complexity (no more than $2^{20}$ key-IV pairs).

*12:17* [Pub][ePrint]
Tradeoff Cryptanalysis of Memory-Hard Functions, by Alex Biryukov and Dmitry Khovratovich
We explore time-memory and other tradeoffs for memory-hard functions, which are supposed to impose significant computational and time penalties if less memory is used than intended. We analyze two schemes: Catena, which has been presented at Asiacrypt 2014, and Lyra2, the fastest finalist of the Password Hashing Competition (PHC).

We demonstrate that Catena\'s proof of tradeoff resilience is flawed, and attack it with a novel \\emph{precomputation tradeoff}. We show that using $M^{2/3}$ memory instead of $M$ we may have no time penalties. We further generalize our method for a wide class of schemes with predictable memory access.

For Lyra2, which addresses memory unpredictability (depending on the input), we develop a novel \\emph{ranking tradeoff} and show how to decrease the time-memory and the time-area product by significant factors. We also generalize the ranking method for a wide class of schemes with unpredictable memory access.

*12:17* [Pub][ePrint]
Leakage-Resilient Cryptography with Key Derived from Sensitive Data, by Konrad Durnoga and Tomasz Kazana and Michał Zając and Maciej Zdanowicz
In this paper we address the problem of large space consumption for protocols in the Bounded Retrieval Model (BRM), which require users to store large secret keys subject to adversarial leakage.We propose a method to derive keys for such protocols on-the-fly from weakly random private data (like text documents or photos, users keep on their disks anyway for non-cryptographic purposes) in such a way that no extra storage is needed. We prove that any leakage-resilient protocol (belonging to a certain, arguably quite broad class) when run with a key obtained this way retains a similar level of security as the original protocol had. Additionally, we guarantee privacy of the data the actual keys are derived from. That is, an adversary can hardly gain any knowledge about the private data except that he could otherwise obtain via leakage. Our reduction works in the Random Oracle model.

As an important tool in the proof we use a newly established bound for min-entropy, which can be of independent interest. It may be viewed as an analogue of the chain rule -- a weaker form of the well-known formula $\\mathbf{H}(X \\vert Y) = \\mathbf{H}(X, Y) - \\mathbf{H}(Y)$ for random variables $X$, $Y$, and Shannon entropy, which our result originates from. For min-entropy only a much more limited version of this relation is known to hold. Namely, the min-entropy of $X$ may decrease by up to the bitlength of $Y$ when $X$ is conditioned on $Y$, in short: $\\widetilde{\\mathbf{H}(X \\vert Y) \\geq \\mathbf{H}_\\infty(X) - \\lvert Y\\rvert$.

In many cases this inequality does not offer tight bounds, and such significant entropy loss makes it inadequate for our particular application. In the quasi chain rule we propose, we inject some carefully crafted side information (spoiling knowledge) to show that with large probability the average min-entropy of $X$ conditioned on both: $Y$ and this side information can be almost lower bounded by the min-entropy of $(X, Y)$ decreased by the min-entropy of $Y$ conditioned on the side information.

*22:31* [Job][New]
Ph.D. student - PUF design and security, *Laboratoire Hubert Curien, University of Lyon, Saint-Etienne, France*
The main objective of the research in the group Applied Cryptography & Telecom is to propose efficient and robust hardware architectures aimed at applied cryptography and telecom that are resistant to passive and active cryptographic attacks. Currently, the central theme of this research consists in designing architectures for Secure Embedded Systems implemented in logic devices such as FPGAs and ASICs. More information on http://laboratoirehubertcurien.fr/spip.php?rubrique29The PhD thesis will take part of the European H2020 Project HECTOR (Hardware Enabled CrypTOgraphy and Randomness) aims at bridging the gap between the mathematics of cryptography with the reality of hardware implementations. HECTOR consortium: Technickon (Austria), KU Leuven (Belgium), Univ. Jean Monnet St-Etienne (France), TU Graz (Austria), THALES Comm. & Security SAS (France), STMicroelectronics Rousset SAS (France), STMicroelectronics SRL (Italia), Brighstsight (Netherlands), Micronic (Slovakia).

Objective of this thesis is to propose a set of tools that would allow simplifying physical unclonable functions (PUFs) design and security assessment.

We are looking for candidates with an outstanding Master in electrical engineering. Strong knowledge in VLSI design, FPGA & ASIC design are required. A first experience with PUF or TRNG design would be also appreciated. Notions of stochastic modelling will be a bonus. Skills in oral presentation and scientific writing in English are required (French is not required but recommended).

The PhD position will start before October 2015, it is funded for 36 months.

*21:17* [Pub][ePrint]
Key Homomorphic PRFs and Their Applications, by Dan Boneh and Kevin Lewi and Hart Montgomery and Ananth Raghunathan
A pseudorandom function F : K x X -> Y is said to be key homomorphic if given F(k1, x) and F(k2, x) there is an efficient algorithm to compute F(k1 xor k2, x), where xor denotes a groupoperation on k1 and k2 such as xor. Key homomorphic PRFs are natural objects to study and have a number of interesting applications: they can simplify the process of rotating encryption keys for encrypted data stored in the cloud, they give one round distributed PRFs, and they can be the basis of a symmetric-key proxy re-encryption scheme. Until now all known constructions

for key homomorphic PRFs were only proven secure in the random oracle model. We construct the first provably secure key homomorphic PRFs in the standard model. Our main construction

is based on the learning with errors (LWE) problem. In the proof of security we need a variant of LWE where query points are non-uniform and we show that this variant is as hard as the standard LWE. We also construct key homomorphic PRFs based on the decision linear assumption in groups with an l-linear map. We leave as an open problem the question of constructing standard model key homomorphic PRFs from more general assumptions.